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Preparation of in-situ 5 vol% TiB2 particulate reinforced Al–4.5Cu alloy matrix composites assisted by improved mechanical stirring process
Preparation of in-situ 5 vol% TiB 2 particulate reinforced Al–4.5Cu alloy matrix composites assisted by improved mechanical stirring process ¨ Xuecheng Duan, Ping An Qi Gao, Shusen Wu, Shulin L U, PII: DOI: Reference:
S0264-1275(16)30024-7 doi: 10.1016/j.matdes.2016.01.023 JMADE 1224
To appear in: Received date: Revised date: Accepted date:
27 August 2015 4 January 2016 7 January 2016
¨ Xuecheng Duan, Ping Please cite this article as: Qi Gao, Shusen Wu, Shulin LU, An, Preparation of in-situ 5 vol% TiB2 particulate reinforced Al–4.5Cu alloy matrix composites assisted by improved mechanical stirring process, (2016), doi: 10.1016/j.matdes.2016.01.023
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Preparation of in-situ 5vol% TiB2 particulate reinforced Al-4.5Cu alloy matrix composites assisted
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by improved mechanical stirring process
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Qi Gaoa, Shusen Wua, Shulin LÜ*a, Xuecheng Duana, Ping Ana State Key Lab of Materials Processing and Die & Mould Technology, Huazhong University of Science
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and Technology, 1037 Luoyu Road, Wuhan 430074, China
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*Corresponding author email: [email protected]
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Abstract: In-situ 5vol% TiB2/Al-4.5Cu composites are successfully prepared by the salt-metal reactions
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assisted by improved mechanical stirring. The X-ray diffraction confirms the formation of TiB2 particles, and no cluster or large precipitate of intermediate phases Al3Ti and AlB2 can be traced. Mechanical
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stirring can reduce the large agglomerations and finally eliminate them when the speed rises to 540 rpm. Most of TiB2 particles are uniformly distributed along the grain boundary in the matrix, and lots of
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dispersed particles with a size under 0.4 μm are also found. Yield stress and ultimate tensile strength of the composite are improved by 85 % and 46 %, respectively.
Keywords: Al-matrix composites; Agglomerations; Particle size; TiB2; Mechanical stirring
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1.
INTRODUCTION
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The particulates reinforced aluminum matrix composites (PRAMCs) have outstanding combination of
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mechanical properties, such as high specific strength, specific modulus, hardness and low thermal
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expansion coefficient [1-4]. They are promising materials for the application in structure, transportation, aerospace and the military [1-5]. Among various potential particulate reinforcements such as B4C, SiC,
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Si3N4, TiC, TiB2, Al2O3 [5-10], TiB2 phase is portrayed to be an outstanding reinforcements in aluminum because of its good thermodynamic stability, high hardness, high melting point, high modulus, high
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corrosion resistance and low density [5, 11-14]. Among various aluminum alloys, Al-Cu alloys have
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relative high strength, high hardness and high heat resistance, but low corrosion resistance. Therefore the
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introduction of high hardness TiB2 particles into Al-Cu alloy has been studied for better mechanical properties and corrosion resistance of Al-Cu alloys [15-17].
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The PRAMCs synthesized by in-situ processes have clearer interface between matrix and reinforcements, and better matrix-reinforcements interfacial thermodynamic stability compared with
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exogenously-formed processes. In the situation of TiB2 particulate reinforced aluminum matrix composites, the salt-metal reaction route is widely used to synthesis the in-situ TiB2 particles for its advantages compared with other in-situ processes, namely: (a) the reaction temperature is lower than that in other reaction systems, and the size of the TiB 2 particles can be submicron; (b) the TiB2 particles are formed by reactions in the melt and without wetting problem; (c) the matrix-reinforcements interface is clean and stable; (d) bulk and continuous casting of composites are possible. The following sequence is the exothermic process to form the TiB2 particles [15, 18]: (1)
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(2)
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(3)
(4)
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And the TiB2 particles can also be formed by a direct reaction [19]:
But the salt-metal reaction route also has its limitation. First, the utilization rate of Ti and B in K2TiF6
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and KBF4 is low. So the weight ratio of salts needs to be very high to fabricate high TiB2 particulate volume percentage aluminum matrix composites. Molten salts are floating on top of the melt, and the
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thick layer of molten melt inhibits the subsequent reaction. Second, TiB 2 agglomerations increase with the
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increasing of particulate volume percentage. And that harms the mechanical properties of the composites
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[20]. But higher volume percentage of dispersed particles bring more effective reinforcement according to Orowan strengthening [21]. So it is necessary to solve the problems mentioned above to fabricate
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uniformly distributed high TiB2 particulate volume percentage aluminum matrix composites. But researches about TiB2 particles reinforced Al-Cu alloy composites with high particle volume percentage is
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scarcer so far.
Mechanical stirring is an easy way to improve the distribution of TiB2 particles. But low mechanical stirring speed cannot effectively uniform the distribution of particles and high mechanical stirring speed increases hydrogen absorption and oxidation [22]. The depth where mechanical stirring works is also important, that mechanical stirring at the interface between molten salts and melt easily brings reaction products into the melt [22]. In this work, the in-situ 5vol% TiB2/Al-4.5Cu composites are successfully synthesized through the salt-metal reaction route with mechanical stirring during the reaction holding. The mechanical stirring is
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carried out below the interface between molten salts and melt to solve the problems about mechanical
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stirring mentioned above. The large TiB2 agglomerations are effectively reduced with high speed
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mechanical stirring. The distribution of the TiB2 particles in the Al-Cu matrix and the mechanical
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properties of the in-situ 5vol% TiB2/Al-4.5Cu composites assisted by mechanical stirring are discussed. EXPERIMENTAL PROCEDURE Raw materials and processing
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2.1.
Commercial pure Al (99.8 %, wt%, the same below), and pure Cu (99.9 %) were used to prepare the
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Al-4.5%Cu base metal. Two kind of chemically pure salts, potassium fluotitanate (K2TiF6) and potassium
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fluoborate (KBF4) were used to synthesize the TiB2 reinforcement phase. Specific flux contains MgF2 and
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Na3AlF6 was also used to help the reactions.
Aluminum ingot was melted in a graphite crucible using a resistance furnace at first, and the Cu chips
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for preparation of the base metal was added into the melt at 700 ℃. Then the melt was over heated up to 830 ℃. The K2TiF6 and KBF4 salts, thoroughly mixed with specific flux, were gradually added into the
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melt after preheated to 300 ℃ in 2h. Two kinds of salt powders are mixed with a molar ratio of Ti/B = 1/2, and the total mass of salts is set to synthesize 5vol% (nominal) TiB2 particles. During adding the salts, the temperature of the melt was controlled below 830 ℃ and avoid great fluctuation of the reaction temperature. The reaction holding time was set for 30 min [23]. Mechanical stirring was introduced by a graphite stirring bar during the reaction holding. The stirring bar was set below the interface of the melt and salts as shown in Fig. 1, and the stirring speed was set at 0, 180, 360 and 540 rpm, respectively (720 rpm and 1080 rpm had been introduced in former works, the stirring speed higher than 540 rpm caused serious loss of matrix metal and introduced more inclusions. In this work, large agglomerations can be
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effectively eliminated when stirring speed up to 540 rpm, stirring speed higher than 540 rpm brings no
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obviously change on agglomeration elimination. Thus only the results of stirring speed from 0 to 540 rpm
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are discussed in this work). After the holding time of the reaction, the slag was removed and the melt was
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cooled down to 720 ℃. Then the melt was cast into a permanent mould preheated at 200 ℃.
2.2.
Characterization
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Fig. 1 Sketch of mechanical stirring.
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Specimens of the composites for metallographic examination were cut from the ends of tensile test samples, then grinded, polished and etched by 0.5% HF solution. Samples which need deep etching were
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etched by 20% NaOH solution. A SHIMADZU XRD-7000S X-ray diffractometer with Cu Ka radiation operated at 40 kV and 40 mA was used to identify the phases in the composites. A DMM-480C optical microscope and a JEOL JSM-7600F scanning electron microscope (SEM) with an Inca X-Max 50 energy dispersive X-ray analysis were employed to examine the microstructures of the composites. A Tecnai G2 F30 (FEI, Holland) transmission electron microscope was introduced for TEM observation and identification of particles. The room temperature (25 ℃) tensile tests was conducted on a SHIMADZU AG-100KN tester with a 2 mm/min crosshead speed, and the GB/T228.1-2010 was followed. The size of
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tensile test specimen is shown in Fig. 2. The average ultimate tensile strength value was obtained from
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four samples for each specific condition.
RESULTS AND DISCUSSION In-situ formation of TiB2 phase
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3.1.
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Fig. 2 Sketch of tensile test specimen.
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The in-situ TiB2 phase is successfully formed through the aluminothermic reaction of a complex potassium fluoride molten salt mixture. Fig. 3 shows the X-ray diffraction (XRD) patterns of the in-situ
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AMCs (Aluminum Matrix Composites) fabricated on different conditions. The TiB2 peaks can be found in all results, no any other peaks such as Al3Ti or AlB2 peaks can be found. It confirms the formation of TiB2
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phase and no any other intermediate phases can be detected. The intensity of TiB2 peaks in the no flux condition is weaker compared with others due to the loss of elements caused by the gasification of salts and uncompleted reactions. It is worth to note that the intensity of TiB 2 peaks slightly increases with the increasing of stirring speed during the holding, and this indicates that stirring during holding may promote the yield of TiB2 particles. The peaks of Al2Cu, which is a regular intermetallic in Al-Cu alloys, are also found. It is worth to note that no any other peaks of by-products like Al2O3 or K3AlF6 could be found, which proves that high speed stirring below the interface of melt and salts would not introduce any
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by-products into the composites. This is because of stirring below the interface of melt and salts would
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not drastically break up the interface but only agitate the melt.
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Fig. 3 XRD patterns of the in-situ AMCs on different conditions:
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(a). No flux adding, (b). Adding flux without stirring, (c). Adding flux with 180 rpm stirring, (d). Adding
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flux with 360 rpm stirring, (e). Adding flux with 540 rpm stirring. Microstructures
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Fig. 4 shows the difference of molten salts between with and without flux. Due to the mass of salts is more than half the mass of the base metal, the molten salts form a very thick fluid layer on top of the melt during reaction, as shown in Fig. 4a. The thick layer of molten salts not only inhibits the subsequent reaction but also causes some salts get heated before fully contacting with the melt, which may lead to severe gasification and decomposition of the salts and high temperature in local melt. The local high temperature will accelerate the precipitation of intermediate phases at the place where the concentration of Ti or B is higher than the other [24] and further become large agglomerations with different morphologies, and not easy to be eliminated by the reaction holding. Agglomerations are clusters when Ti quantity is exceeded and agglomerations are chain-like when B exceeded, as shown in Fig. 5a [25-27].
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b
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Fig. 4 Sketch of effect of adding specific flux:
(a). Without adding flux, (b). With adding flux.
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After adding specific flux, the MgF2 in the flux is not fully melted and the viscosity of molten salt is
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increased. Then the molten salts wrap up the melt instead of floating on top, as shown in Fig. 4b, which is
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verified after pouring of the melt. Thus the reaction area is expanded and the salts become easier to fully contact with the melt. The yield of TiB2 particles is increased and large agglomerations caused by local
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high temperature and element concentration are reduced after adding the flux, as shown in Fig. 5b.
Reduced agglomerations
Chain-like agglomeration Cluster agglomerations
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200μm
Fig. 5 Influence of adding flux on reducing large agglomerations: (a). Clusters and chain-like agglomerations when no flux adding, (b). Reduced agglomerations with adding of flux.
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b
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Large agglomerations
Decreased large agglomerations
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c
Broken agglomeration
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200μm
Small agglomerations
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Fig. 6 Microstructures of large agglomerations in the in-situ AMCs:
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(a). Without stirring, (b). With stirring at 180 rpm,
(c). With stirring at 360 rpm, (d). With stirring at 540 rpm.
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Adding flux can help the reaction and reduce large agglomerations, but it cannot eliminate those large agglomerations. Therefore, the mechanical stirring is introduced during the reaction holding to improve the distribution of TiB2 particles. Fig. 6 shows the largest agglomeration found in composites. When there is no mechanical stirring during the reaction holding, some large agglomerations over 200 μm can be found in Fig. 6a. These agglomerations cannot disappear just by holding for reaction. In the situation of mechanical stirring at 180 rpm, the agitation promotes the reactions and large agglomerations start to decrease. But the slow stirring speed cannot effectively break the agglomerations and a few large agglomerations larger than 200 μm still can be found in Fig. 6b. When the speed of mechanical stirring
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rises to 360 rpm, large agglomerations are effectively broken up to small agglomerations by intense
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agitation, as shown in Fig. 6c. But there is still many agglomerations in the matrix, thus 540 rpm
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mechanical stirring is applied. Large agglomerations in composite in situation of 540 rpm mechanical
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stirring have a significant change. With the more intense agitation, the large agglomerations are not only broken up to smaller agglomerations but also be eliminated by the promoted reactions. Only very few
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small agglomerations under 50 μm can be found in the matrix, as shown in Fig. 6d.
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Fig. 7 Microstructures of particle distribution of the in-situ AMCs: (a). Without stirring, (b). With stirring at 180 rpm, (c). With stirring at 360 rpm, (d). With stirring at 540 rpm. Fig. 7 shows the distribution of TiB2 particles in the in-situ AMCs. It clearly shows that most of TiB2 particles are distributed along the grain boundary regions on all situations, since they are rejected by the
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growing solid phase at the solid/liquid interface during solidification [28, 29]. When there is no
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mechanical stirring during the reaction holding, TiB 2 particles are not uniformly distributed in the matrix
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and small agglomerations can be found in the grain boundary regions. This is due to the low yield of TiB2
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particles and nonuniform in melt. With the mechanical stirring at 180 rpm, the yield of TiB2 particles increases and distribution of TiB2 particles become uniform, the small agglomerations are also reduced by
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agitation. When the speed of mechanical stirring rises to 360 rpm, the small agglomerations are further reduced by intense agitation. But the grain boundary regions are greatly increased since grains are more
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refined, and the distribution of TiB2 particles become less uniform due to the limited amount. When the
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speed of mechanical stirring rises to 540 rpm, more TiB2 particles distributed along the grain boundary
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with the elimination of large agglomerations and more completed reactions. The distribution of TiB 2
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A
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particles become very uniform again with the more intense agitation and further growth of grains.
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Fig. 8 EDX mapping result of the agglomerations along the grain boundary regions. Fig. 8 shows the EDX mapping result of the agglomerations along the grain boundary regions, and marker A indicates the gathering of particles along the grain boundary. Although most TiB2 particles gather along the grain boundary regions, it is hard to find clear distinct particulate contours in the
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agglomerations along the grain boundary, as shown in Fig. 8. From the mapping results, elements Ti and
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B are both existed in the agglomerations. The distributions of Ti and B in the agglomeration are nearly the
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same, which indicates that agglomerations are mainly formed by compound of Ti and B. The element Cu
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is also found in the agglomerations. Combining the XRD result, it indicates that the Al2Cu phase may wraps up the TiB2 particles since the Al2Cu phase in the binary Al-Cu alloy has a strong tendency to wet
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the TiB2 particles [15].
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b
Tiny particles
Larger particles
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Tiny particles
Tiny particles
d Larger particles
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c
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Larger particles
Tiny particles
Larger particles
Fig. 9 SEM micrographs of the in-situ AMCs formed after deep etching: (a). Without stirring, (b). With stirring at 180 rpm, (c). With stirring at 360 rpm, (d). With stirring at 540 rpm.
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For a further study on the formation of agglomerations along the grain boundary, samples after deep
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etching are observed by SEM. Fig. 9 shows the results on different conditions. A kind of extremely tiny
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particles, with sizes of 10-100 nm, is found in agglomerations along the grain boundary in all different
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conditions. These particles adhere to the larger ones. They may be formed by the residual Ti and B in the liquid phase which is consumed at the final stage of solidification or directly precipitate from those liquid
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phase. These extremely tiny particles are then analyzed by TEM, and Fig. 10 shows the results. The extremely tiny particles have no typical hexagonal morphology, and maybe due to the uncompleted growth
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for a crystal. The well bonded extremely tiny particles have a clear interface with the matrix. TiB2 crystal has a hexagonal structure, and the lattice parameters are a = b = 0.303 nm and c = 0.323 nm. According the
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diffraction pattern, dA = 0.3233 nm, dB= 0.2039 nm, dC= 0.2630 nm, θAB = 51.11 ° and θBC= 39.06 °. These parameters can match the d( 0 0 1 ), d( 1 0 1 ), d( 1 0 0 ), θ( 0 0 1 )//( 1 0 1 ) and θ( 1 0 1 )//( 1 0 0 ) in the TiB2 crystal system.
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Combining with the XRD results, the diffraction pattern further confirms that the extremely tiny particles are TiB2 particles. And larger TiB2 particles with a hexagonal morphology can also be observed in Fig. 9.
b
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5 1/nm
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Fig. 10 TEM results of extremely tiny particles: (a). Morphology of particles, (b). Diffraction pattern of particle A.
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With the evidence above, we presumed a possible explanation for the formation of agglomerations
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along the grain boundary without clear particulate contours. During the solidification, most particles are
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rejected by the growing solid phase at the solid/liquid interface and the liquid Al2Cu phase wets and
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wraps up the TiB2 particles. Then the particles gather in the residual liquid phase at where becomes the grain boundary after solidification. The extremely tiny particles formed from the liquid phase fills the
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gaps of big particles with the Al2Cu phase. Subsequently, the agglomerations without clear particulate contours formed.
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d
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Fig. 11 SEM micrographs of dispersed particles in the in-situ AMCs: (a). Without stirring, (b). With stirring at 180 rpm, (c). With stirring at 360 rpm, (d). With stirring at 540 rpm.
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Although most of the TiB2 particles gather along the grain boundary regions, a lot of dispersed TiB2
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particles still could be found beside agglomerations along the grain boundary, as shown in Fig. 11. These
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particles is well bonded with the matrix. The particle size of most dispersed TiB2 is measured to be under
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0.4 μm in diameter, which is very suitable for Orowan strengthening. And the particle size has no obvious
3.3.
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change with the increase of the stirring speed.
Mechanical properties
Stirring speed
Yield stress
UTS
Elongation
(rpm)
(MPa)
(MPa)
(%)
0
65
166
12.50
AMC
0
82
212
5.65
AMC
180
95
223
6.25
360
114
251
7.05
540
120
243
6.48
AMC
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AMC
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Al-4.5Cu
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Specimens
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Table 1 As-cast mechanical properties of the in-situ AMCs and the matrix metal.
Table 1 shows the results of as-cast mechanical properties of the in-situ AMCs and matrix metal, i.e.,
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Al-4.5%Cu alloy. The yield stress and UTS of composite without mechanical stirring are improved by 26 % and 28 % respectively compared with the matrix metal. The yield stress and UTS of composite with mechanical stirring at 180 rpm are improved by 46 % and 34 % respectively compared with matrix metal. When mechanical stirring speed rises to 360 rpm, the yield stress and UTS of composite are improved by 75 % and 51 % respectively compared with matrix metal. When mechanical stirring speed further rises to 540 rpm, the yield stress and UTS of composite are improved by 85 % and 46 % respectively compared with matrix metal. The ductility of the composites is all reduced compared with the matrix metal. The ductility of the composites increases with the rise of stirring speed when the speed is under 360 rpm and deceases when the stirring speed rises to 540 rpm.
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According to the Hall-Petch relationship [1, 15] and Orowan strengthening mechanism, the
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improvement of yield stress of all composites is attributed to the effectively refined grains and the
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uniformly dispersed TiB2 particles. Fig. 12 shows the average grain size. Grains of all the composites are
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effectively refined. This refinement is mainly attributed to the promotion of heterogeneous nucleation with a little excessed intermediate phase Al3Ti [15] (which formed with different reaction rate between with the presence of
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reaction (1) and (2), and easily consumed by peritectic reaction:
element B [30]), and the hindering of TiB2 particles on the growth of grains. Mechanical stirring can
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promote the reaction (3) and consume the intermediate phase Al3Ti, meanwhile the broken of large
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agglomerations increases the content of unreacted Al 3Ti in the melt. Thus the grain size has a nonlinear
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change with the increasing of mechanical stirring.
Fig. 12 Average grain size of the in-situ AMCs and matrix metal. The yield stress increases with the increasing of mechanical stirring speed, and the gradual elimination of large agglomerations mainly contributes to this trend. Comparing the situation of stirring at 180 rpm with no stirring, although the average grain size slightly increase from 27.3 μm to 31.4 μm due to the agitation promote the reactions and the heterogeneous nuclei reduced, as shown in Fig. 12, the particles distributed more uniform and the large agglomerations are smaller. Thus the yield stress is improved from
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82 MPa to 95 MPa with a stronger Orowan strengthening. Comparing the situation of stirring at 360 rpm
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with stirring at 180 rpm, the large agglomerations are broken up and reduced. So the heterogeneous nuclei
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increased and the moderate agitation is unable to decrease them. Thus the grains are much more refined
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from 31.4 μm to 15.6 μm in average size and the yield stress has an effectively improvement from 95 MPa to 114 MPa. Comparing the situation of stirring at 540 rpm with stirring at 360 rpm, the violent
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agitation not only eliminate the large agglomerations but also reduce the heterogeneous nuclei by greatly promotion of the reaction (3). So the grains grow to 30.2 μm in average size. But more uniformly
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dispersed TiB2 particles contribute to stronger Orowan strengthening after elimination of large
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agglomerations. Thus the yield stress still has a little improvement from 114 MPa to 120 MPa.
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The improvement of UTS of all composites may be attributed to the gain boundary strengthening compared with matrix metal. When gathering along the grain boundary, TiB 2 particles cannot contribute
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to Orowan strengthening. But they can directly hinder the dislocation slip since they have high elastic modulus and high hardness. The CTE (Coefficient of Thermal Expansion) mismatch between these
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particles with matrix also brings dislocation strengthening at grain boundary, according to CTE mismatch strengthening [1, 31].The extremely tiny particles fill in the gaps of large particles and improve the gain boundary strengthening. Comparing the situation of stirring at 180 rpm with no stirring, small agglomerations are reduced and more TiB2 particles uniformly distribute along the grain boundary. So the UTS is still a little improved with a slightly bigger average grain size. Comparing the situation of stirring at 360 rpm with stirring at 180 rpm, the much more refined grains greatly increase the grain boundary regions. And uniformly distributed particles along the grain boundary contribute to an obviously improvement of UTS from 223 MPa to 251 MPa. Comparing the situation of stirring at 540 rpm with
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stirring at 360 rpm, although the distribution of TiB 2 particles along the grain boundary becomes more
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uniform, the bigger grains decreases the grain boundary regions and affects the UTS. The UTS is slightly
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lower. The elimination of large agglomerations and the uniform distribution of TiB2 particles along the
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grain boundary effectively improve the UTS, as the UTS of the composite with stirring at 540 rpm is improved by 20 MPa compared with the composite with stirring at 180 rpm, which have a similar average
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grain size.
The ductility of all composites is lower than the matrix metal due to the hindering of TiB 2 particles
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on the deformation of grains. But with uniform distribution of TiB 2 particles and reducing of large
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CONCLUSIONS
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agglomerations, the ductility can be improved.
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The 5vol% TiB2/Al-4.5Cu composites are successfully prepared with mechanical stirring. TiB2 particles are uniformly distributed without cluster or large precipitate of intermediate phases Al3Ti and
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AlB2. The size of TiB2 particles is under 0.4 μm in diameter. The grains are effectively refined. With the mechanical stirring below the interface of melt and salts during the reaction holding, the introduction of by-products like Al2O3 or K3AlF6 into the composites is effectively avoided. The large agglomerations are effectively reduced by mechanical stirring and finally eliminated when the speed of mechanical stirring rises to 540 rpm. The distribution of TiB2 particles becomes more uniform with the increasing of stirring speed.
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The formation of agglomerations along the grain boundary without clear particulate contours is
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explained. The extremely tiny particles fill the gaps of large particles with the Al2Cu phase which wet
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the TiB2 particles.
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The as-cast mechanical properties of TiB2/Al-4.5Cu composites have a great improvement compared with the matrix metal. The yield stress of composites has been improved by 26 %, 46 %, 75 % and 85 %
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with the mechanical stirring at 0, 180, 360 and 540 rpm, respectively, and the improvements of UTS is
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28 %, 34 %, 51 % and 46 %, respectively.
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AKNOWLEDGMENTS
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This work was financially supported by the National Basic Research Program of China (No. 2012CB619600), the Fundamental Research Funds for the Central Universities, HUST (No.
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2014QNRC003, No. 2015QN010), and the Research Project of State Key Laboratory of Materials Processing and Die & Mould Technology. The authors would also like to express their appreciation to
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Analytical and Testing Center, HUST.
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[12] K.L. Tee, L. Lu, M.O. Lai. Improvement in mechanical properties of in-situ Al-TiB2 composite by incorporation of carbon. Mater. Sci. Eng., A 2003; 339: 227-231. [13] L. Lu, M.O. Lai, Y. Su, H.L. Teo, C.F. Feng. In situ TiB2 reinforced Al alloy composites. Scripta. Mater. 2001; 45: 1017-1023. [14] C.S. Ramesh, S. Pramod, R. Keshavamurthy. A study on microstructure and mechanical properties of Al 6061-TiB2 in-situ composites. Mater. Sci. Eng., A 2011; 528: 4125-4132. [15] L. Lu, M.O. Lai, F.L. Chen. Al-4wt% Cu composite reinforced with in-situ TiB2 particles. Acta. Mater. 1997; 45: 4297-4309.
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[17] J. Xue, Y. Han, J. Wang, B. Sun. Study on squeeze casting of an in situ 5 vol.-% TiB2/2014 Al
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[19] Z. Liu, M. Rakita, W. Xu, X. Wang, Q. Han. Ultrasound assisted salts-metal reaction for synthesizing TiB2 particles at low temperature. Chem. Eng. J. 2015; 263: 317-324.
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[22] F. Chen, Z.N. Chen, F. Mao, T.M. Wang, Z.Q. Cao. TiB2 reinforced aluminum based in situ composites fabricated by stir casting. Mater. Sci. Eng., A 2015; 625: 357-368. [23] Q. Gao, S.S. Wu, S.L.LÜ, X.C. Duan. Microstructure and properties of in-situ TiB2 particle reinforced Al-4.5Cu composites. In: 20th International Conference on Composite materials. Copenhagen, July, 2015. [24] J. Fjellstedt, A.E.W. Jarfors, T. El-Benawy. Experimental investigation and thermodynamic assessment of the Al-rich side of the Al-B system. Mater. Des. 2001; 22: 443-449.
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aluminum. J. Alloys. Compd. 2015; 622: 831-836.
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[28] S.S. Wu, H. Nakae. Nucleation effect of alumina in Al-Si/Al2O3 composites. J. Mater. Sci. Lett.
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Graphical abstract
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Highlights
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1. No by-products were introduced into the melt by mechanical stirring below interface.
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2. Large agglomerations were eliminated in high TiB2 volume percentage composites.
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3. Tiny TiB2 particles with a size of 10-100nm were found, and analyzed by TEM.
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S0264-1275(16)30024-7 doi: 10.1016/j.matdes.2016.01.023 JMADE 1224
To appear in: Received date: Revised date: Accepted date:
27 August 2015 4 January 2016 7 January 2016
¨ Xuecheng Duan, Ping Please cite this article as: Qi Gao, Shusen Wu, Shulin LU, An, Preparation of in-situ 5 vol% TiB2 particulate reinforced Al–4.5Cu alloy matrix composites assisted by improved mechanical stirring process, (2016), doi: 10.1016/j.matdes.2016.01.023
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Preparation of in-situ 5vol% TiB2 particulate reinforced Al-4.5Cu alloy matrix composites assisted
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by improved mechanical stirring process
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Qi Gaoa, Shusen Wua, Shulin LÜ*a, Xuecheng Duana, Ping Ana State Key Lab of Materials Processing and Die & Mould Technology, Huazhong University of Science
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and Technology, 1037 Luoyu Road, Wuhan 430074, China
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*Corresponding author email: [email protected]
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Abstract: In-situ 5vol% TiB2/Al-4.5Cu composites are successfully prepared by the salt-metal reactions
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assisted by improved mechanical stirring. The X-ray diffraction confirms the formation of TiB2 particles, and no cluster or large precipitate of intermediate phases Al3Ti and AlB2 can be traced. Mechanical
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stirring can reduce the large agglomerations and finally eliminate them when the speed rises to 540 rpm. Most of TiB2 particles are uniformly distributed along the grain boundary in the matrix, and lots of
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dispersed particles with a size under 0.4 μm are also found. Yield stress and ultimate tensile strength of the composite are improved by 85 % and 46 %, respectively.
Keywords: Al-matrix composites; Agglomerations; Particle size; TiB2; Mechanical stirring
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1.
INTRODUCTION
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The particulates reinforced aluminum matrix composites (PRAMCs) have outstanding combination of
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mechanical properties, such as high specific strength, specific modulus, hardness and low thermal
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expansion coefficient [1-4]. They are promising materials for the application in structure, transportation, aerospace and the military [1-5]. Among various potential particulate reinforcements such as B4C, SiC,
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Si3N4, TiC, TiB2, Al2O3 [5-10], TiB2 phase is portrayed to be an outstanding reinforcements in aluminum because of its good thermodynamic stability, high hardness, high melting point, high modulus, high
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corrosion resistance and low density [5, 11-14]. Among various aluminum alloys, Al-Cu alloys have
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relative high strength, high hardness and high heat resistance, but low corrosion resistance. Therefore the
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introduction of high hardness TiB2 particles into Al-Cu alloy has been studied for better mechanical properties and corrosion resistance of Al-Cu alloys [15-17].
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The PRAMCs synthesized by in-situ processes have clearer interface between matrix and reinforcements, and better matrix-reinforcements interfacial thermodynamic stability compared with
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exogenously-formed processes. In the situation of TiB2 particulate reinforced aluminum matrix composites, the salt-metal reaction route is widely used to synthesis the in-situ TiB2 particles for its advantages compared with other in-situ processes, namely: (a) the reaction temperature is lower than that in other reaction systems, and the size of the TiB 2 particles can be submicron; (b) the TiB2 particles are formed by reactions in the melt and without wetting problem; (c) the matrix-reinforcements interface is clean and stable; (d) bulk and continuous casting of composites are possible. The following sequence is the exothermic process to form the TiB2 particles [15, 18]: (1)
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(2)
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(3)
(4)
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And the TiB2 particles can also be formed by a direct reaction [19]:
But the salt-metal reaction route also has its limitation. First, the utilization rate of Ti and B in K2TiF6
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and KBF4 is low. So the weight ratio of salts needs to be very high to fabricate high TiB2 particulate volume percentage aluminum matrix composites. Molten salts are floating on top of the melt, and the
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thick layer of molten melt inhibits the subsequent reaction. Second, TiB 2 agglomerations increase with the
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increasing of particulate volume percentage. And that harms the mechanical properties of the composites
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[20]. But higher volume percentage of dispersed particles bring more effective reinforcement according to Orowan strengthening [21]. So it is necessary to solve the problems mentioned above to fabricate
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uniformly distributed high TiB2 particulate volume percentage aluminum matrix composites. But researches about TiB2 particles reinforced Al-Cu alloy composites with high particle volume percentage is
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scarcer so far.
Mechanical stirring is an easy way to improve the distribution of TiB2 particles. But low mechanical stirring speed cannot effectively uniform the distribution of particles and high mechanical stirring speed increases hydrogen absorption and oxidation [22]. The depth where mechanical stirring works is also important, that mechanical stirring at the interface between molten salts and melt easily brings reaction products into the melt [22]. In this work, the in-situ 5vol% TiB2/Al-4.5Cu composites are successfully synthesized through the salt-metal reaction route with mechanical stirring during the reaction holding. The mechanical stirring is
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carried out below the interface between molten salts and melt to solve the problems about mechanical
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stirring mentioned above. The large TiB2 agglomerations are effectively reduced with high speed
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mechanical stirring. The distribution of the TiB2 particles in the Al-Cu matrix and the mechanical
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properties of the in-situ 5vol% TiB2/Al-4.5Cu composites assisted by mechanical stirring are discussed. EXPERIMENTAL PROCEDURE Raw materials and processing
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2.1.
Commercial pure Al (99.8 %, wt%, the same below), and pure Cu (99.9 %) were used to prepare the
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Al-4.5%Cu base metal. Two kind of chemically pure salts, potassium fluotitanate (K2TiF6) and potassium
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fluoborate (KBF4) were used to synthesize the TiB2 reinforcement phase. Specific flux contains MgF2 and
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Na3AlF6 was also used to help the reactions.
Aluminum ingot was melted in a graphite crucible using a resistance furnace at first, and the Cu chips
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for preparation of the base metal was added into the melt at 700 ℃. Then the melt was over heated up to 830 ℃. The K2TiF6 and KBF4 salts, thoroughly mixed with specific flux, were gradually added into the
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melt after preheated to 300 ℃ in 2h. Two kinds of salt powders are mixed with a molar ratio of Ti/B = 1/2, and the total mass of salts is set to synthesize 5vol% (nominal) TiB2 particles. During adding the salts, the temperature of the melt was controlled below 830 ℃ and avoid great fluctuation of the reaction temperature. The reaction holding time was set for 30 min [23]. Mechanical stirring was introduced by a graphite stirring bar during the reaction holding. The stirring bar was set below the interface of the melt and salts as shown in Fig. 1, and the stirring speed was set at 0, 180, 360 and 540 rpm, respectively (720 rpm and 1080 rpm had been introduced in former works, the stirring speed higher than 540 rpm caused serious loss of matrix metal and introduced more inclusions. In this work, large agglomerations can be
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effectively eliminated when stirring speed up to 540 rpm, stirring speed higher than 540 rpm brings no
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obviously change on agglomeration elimination. Thus only the results of stirring speed from 0 to 540 rpm
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are discussed in this work). After the holding time of the reaction, the slag was removed and the melt was
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cooled down to 720 ℃. Then the melt was cast into a permanent mould preheated at 200 ℃.
2.2.
Characterization
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Fig. 1 Sketch of mechanical stirring.
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Specimens of the composites for metallographic examination were cut from the ends of tensile test samples, then grinded, polished and etched by 0.5% HF solution. Samples which need deep etching were
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etched by 20% NaOH solution. A SHIMADZU XRD-7000S X-ray diffractometer with Cu Ka radiation operated at 40 kV and 40 mA was used to identify the phases in the composites. A DMM-480C optical microscope and a JEOL JSM-7600F scanning electron microscope (SEM) with an Inca X-Max 50 energy dispersive X-ray analysis were employed to examine the microstructures of the composites. A Tecnai G2 F30 (FEI, Holland) transmission electron microscope was introduced for TEM observation and identification of particles. The room temperature (25 ℃) tensile tests was conducted on a SHIMADZU AG-100KN tester with a 2 mm/min crosshead speed, and the GB/T228.1-2010 was followed. The size of
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tensile test specimen is shown in Fig. 2. The average ultimate tensile strength value was obtained from
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four samples for each specific condition.
RESULTS AND DISCUSSION In-situ formation of TiB2 phase
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Fig. 2 Sketch of tensile test specimen.
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The in-situ TiB2 phase is successfully formed through the aluminothermic reaction of a complex potassium fluoride molten salt mixture. Fig. 3 shows the X-ray diffraction (XRD) patterns of the in-situ
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AMCs (Aluminum Matrix Composites) fabricated on different conditions. The TiB2 peaks can be found in all results, no any other peaks such as Al3Ti or AlB2 peaks can be found. It confirms the formation of TiB2
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phase and no any other intermediate phases can be detected. The intensity of TiB2 peaks in the no flux condition is weaker compared with others due to the loss of elements caused by the gasification of salts and uncompleted reactions. It is worth to note that the intensity of TiB 2 peaks slightly increases with the increasing of stirring speed during the holding, and this indicates that stirring during holding may promote the yield of TiB2 particles. The peaks of Al2Cu, which is a regular intermetallic in Al-Cu alloys, are also found. It is worth to note that no any other peaks of by-products like Al2O3 or K3AlF6 could be found, which proves that high speed stirring below the interface of melt and salts would not introduce any
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by-products into the composites. This is because of stirring below the interface of melt and salts would
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not drastically break up the interface but only agitate the melt.
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Fig. 3 XRD patterns of the in-situ AMCs on different conditions:
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(a). No flux adding, (b). Adding flux without stirring, (c). Adding flux with 180 rpm stirring, (d). Adding
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flux with 360 rpm stirring, (e). Adding flux with 540 rpm stirring. Microstructures
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Fig. 4 shows the difference of molten salts between with and without flux. Due to the mass of salts is more than half the mass of the base metal, the molten salts form a very thick fluid layer on top of the melt during reaction, as shown in Fig. 4a. The thick layer of molten salts not only inhibits the subsequent reaction but also causes some salts get heated before fully contacting with the melt, which may lead to severe gasification and decomposition of the salts and high temperature in local melt. The local high temperature will accelerate the precipitation of intermediate phases at the place where the concentration of Ti or B is higher than the other [24] and further become large agglomerations with different morphologies, and not easy to be eliminated by the reaction holding. Agglomerations are clusters when Ti quantity is exceeded and agglomerations are chain-like when B exceeded, as shown in Fig. 5a [25-27].
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Fig. 4 Sketch of effect of adding specific flux:
(a). Without adding flux, (b). With adding flux.
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After adding specific flux, the MgF2 in the flux is not fully melted and the viscosity of molten salt is
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increased. Then the molten salts wrap up the melt instead of floating on top, as shown in Fig. 4b, which is
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verified after pouring of the melt. Thus the reaction area is expanded and the salts become easier to fully contact with the melt. The yield of TiB2 particles is increased and large agglomerations caused by local
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high temperature and element concentration are reduced after adding the flux, as shown in Fig. 5b.
Reduced agglomerations
Chain-like agglomeration Cluster agglomerations
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200μm
Fig. 5 Influence of adding flux on reducing large agglomerations: (a). Clusters and chain-like agglomerations when no flux adding, (b). Reduced agglomerations with adding of flux.
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Large agglomerations
Decreased large agglomerations
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c
Broken agglomeration
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200μm
Small agglomerations
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Fig. 6 Microstructures of large agglomerations in the in-situ AMCs:
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(a). Without stirring, (b). With stirring at 180 rpm,
(c). With stirring at 360 rpm, (d). With stirring at 540 rpm.
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Adding flux can help the reaction and reduce large agglomerations, but it cannot eliminate those large agglomerations. Therefore, the mechanical stirring is introduced during the reaction holding to improve the distribution of TiB2 particles. Fig. 6 shows the largest agglomeration found in composites. When there is no mechanical stirring during the reaction holding, some large agglomerations over 200 μm can be found in Fig. 6a. These agglomerations cannot disappear just by holding for reaction. In the situation of mechanical stirring at 180 rpm, the agitation promotes the reactions and large agglomerations start to decrease. But the slow stirring speed cannot effectively break the agglomerations and a few large agglomerations larger than 200 μm still can be found in Fig. 6b. When the speed of mechanical stirring
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rises to 360 rpm, large agglomerations are effectively broken up to small agglomerations by intense
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agitation, as shown in Fig. 6c. But there is still many agglomerations in the matrix, thus 540 rpm
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mechanical stirring is applied. Large agglomerations in composite in situation of 540 rpm mechanical
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stirring have a significant change. With the more intense agitation, the large agglomerations are not only broken up to smaller agglomerations but also be eliminated by the promoted reactions. Only very few
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small agglomerations under 50 μm can be found in the matrix, as shown in Fig. 6d.
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50μm
50μm
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Fig. 7 Microstructures of particle distribution of the in-situ AMCs: (a). Without stirring, (b). With stirring at 180 rpm, (c). With stirring at 360 rpm, (d). With stirring at 540 rpm. Fig. 7 shows the distribution of TiB2 particles in the in-situ AMCs. It clearly shows that most of TiB2 particles are distributed along the grain boundary regions on all situations, since they are rejected by the
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growing solid phase at the solid/liquid interface during solidification [28, 29]. When there is no
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mechanical stirring during the reaction holding, TiB 2 particles are not uniformly distributed in the matrix
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and small agglomerations can be found in the grain boundary regions. This is due to the low yield of TiB2
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particles and nonuniform in melt. With the mechanical stirring at 180 rpm, the yield of TiB2 particles increases and distribution of TiB2 particles become uniform, the small agglomerations are also reduced by
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agitation. When the speed of mechanical stirring rises to 360 rpm, the small agglomerations are further reduced by intense agitation. But the grain boundary regions are greatly increased since grains are more
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refined, and the distribution of TiB2 particles become less uniform due to the limited amount. When the
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speed of mechanical stirring rises to 540 rpm, more TiB2 particles distributed along the grain boundary
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with the elimination of large agglomerations and more completed reactions. The distribution of TiB 2
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A
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particles become very uniform again with the more intense agitation and further growth of grains.
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Fig. 8 EDX mapping result of the agglomerations along the grain boundary regions. Fig. 8 shows the EDX mapping result of the agglomerations along the grain boundary regions, and marker A indicates the gathering of particles along the grain boundary. Although most TiB2 particles gather along the grain boundary regions, it is hard to find clear distinct particulate contours in the
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agglomerations along the grain boundary, as shown in Fig. 8. From the mapping results, elements Ti and
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B are both existed in the agglomerations. The distributions of Ti and B in the agglomeration are nearly the
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same, which indicates that agglomerations are mainly formed by compound of Ti and B. The element Cu
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is also found in the agglomerations. Combining the XRD result, it indicates that the Al2Cu phase may wraps up the TiB2 particles since the Al2Cu phase in the binary Al-Cu alloy has a strong tendency to wet
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the TiB2 particles [15].
a
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b
Tiny particles
Larger particles
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Tiny particles
Tiny particles
d Larger particles
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Larger particles
Tiny particles
Larger particles
Fig. 9 SEM micrographs of the in-situ AMCs formed after deep etching: (a). Without stirring, (b). With stirring at 180 rpm, (c). With stirring at 360 rpm, (d). With stirring at 540 rpm.
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For a further study on the formation of agglomerations along the grain boundary, samples after deep
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etching are observed by SEM. Fig. 9 shows the results on different conditions. A kind of extremely tiny
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particles, with sizes of 10-100 nm, is found in agglomerations along the grain boundary in all different
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conditions. These particles adhere to the larger ones. They may be formed by the residual Ti and B in the liquid phase which is consumed at the final stage of solidification or directly precipitate from those liquid
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phase. These extremely tiny particles are then analyzed by TEM, and Fig. 10 shows the results. The extremely tiny particles have no typical hexagonal morphology, and maybe due to the uncompleted growth
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for a crystal. The well bonded extremely tiny particles have a clear interface with the matrix. TiB2 crystal has a hexagonal structure, and the lattice parameters are a = b = 0.303 nm and c = 0.323 nm. According the
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diffraction pattern, dA = 0.3233 nm, dB= 0.2039 nm, dC= 0.2630 nm, θAB = 51.11 ° and θBC= 39.06 °. These parameters can match the d( 0 0 1 ), d( 1 0 1 ), d( 1 0 0 ), θ( 0 0 1 )//( 1 0 1 ) and θ( 1 0 1 )//( 1 0 0 ) in the TiB2 crystal system.
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Combining with the XRD results, the diffraction pattern further confirms that the extremely tiny particles are TiB2 particles. And larger TiB2 particles with a hexagonal morphology can also be observed in Fig. 9.
b
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Fig. 10 TEM results of extremely tiny particles: (a). Morphology of particles, (b). Diffraction pattern of particle A.
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With the evidence above, we presumed a possible explanation for the formation of agglomerations
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along the grain boundary without clear particulate contours. During the solidification, most particles are
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rejected by the growing solid phase at the solid/liquid interface and the liquid Al2Cu phase wets and
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wraps up the TiB2 particles. Then the particles gather in the residual liquid phase at where becomes the grain boundary after solidification. The extremely tiny particles formed from the liquid phase fills the
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gaps of big particles with the Al2Cu phase. Subsequently, the agglomerations without clear particulate contours formed.
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Fig. 11 SEM micrographs of dispersed particles in the in-situ AMCs: (a). Without stirring, (b). With stirring at 180 rpm, (c). With stirring at 360 rpm, (d). With stirring at 540 rpm.
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Although most of the TiB2 particles gather along the grain boundary regions, a lot of dispersed TiB2
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particles still could be found beside agglomerations along the grain boundary, as shown in Fig. 11. These
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particles is well bonded with the matrix. The particle size of most dispersed TiB2 is measured to be under
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0.4 μm in diameter, which is very suitable for Orowan strengthening. And the particle size has no obvious
3.3.
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change with the increase of the stirring speed.
Mechanical properties
Stirring speed
Yield stress
UTS
Elongation
(rpm)
(MPa)
(MPa)
(%)
0
65
166
12.50
AMC
0
82
212
5.65
AMC
180
95
223
6.25
360
114
251
7.05
540
120
243
6.48
AMC
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AMC
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Al-4.5Cu
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Specimens
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Table 1 As-cast mechanical properties of the in-situ AMCs and the matrix metal.
Table 1 shows the results of as-cast mechanical properties of the in-situ AMCs and matrix metal, i.e.,
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Al-4.5%Cu alloy. The yield stress and UTS of composite without mechanical stirring are improved by 26 % and 28 % respectively compared with the matrix metal. The yield stress and UTS of composite with mechanical stirring at 180 rpm are improved by 46 % and 34 % respectively compared with matrix metal. When mechanical stirring speed rises to 360 rpm, the yield stress and UTS of composite are improved by 75 % and 51 % respectively compared with matrix metal. When mechanical stirring speed further rises to 540 rpm, the yield stress and UTS of composite are improved by 85 % and 46 % respectively compared with matrix metal. The ductility of the composites is all reduced compared with the matrix metal. The ductility of the composites increases with the rise of stirring speed when the speed is under 360 rpm and deceases when the stirring speed rises to 540 rpm.
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According to the Hall-Petch relationship [1, 15] and Orowan strengthening mechanism, the
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improvement of yield stress of all composites is attributed to the effectively refined grains and the
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uniformly dispersed TiB2 particles. Fig. 12 shows the average grain size. Grains of all the composites are
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effectively refined. This refinement is mainly attributed to the promotion of heterogeneous nucleation with a little excessed intermediate phase Al3Ti [15] (which formed with different reaction rate between with the presence of
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reaction (1) and (2), and easily consumed by peritectic reaction:
element B [30]), and the hindering of TiB2 particles on the growth of grains. Mechanical stirring can
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promote the reaction (3) and consume the intermediate phase Al3Ti, meanwhile the broken of large
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agglomerations increases the content of unreacted Al 3Ti in the melt. Thus the grain size has a nonlinear
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change with the increasing of mechanical stirring.
Fig. 12 Average grain size of the in-situ AMCs and matrix metal. The yield stress increases with the increasing of mechanical stirring speed, and the gradual elimination of large agglomerations mainly contributes to this trend. Comparing the situation of stirring at 180 rpm with no stirring, although the average grain size slightly increase from 27.3 μm to 31.4 μm due to the agitation promote the reactions and the heterogeneous nuclei reduced, as shown in Fig. 12, the particles distributed more uniform and the large agglomerations are smaller. Thus the yield stress is improved from
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82 MPa to 95 MPa with a stronger Orowan strengthening. Comparing the situation of stirring at 360 rpm
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with stirring at 180 rpm, the large agglomerations are broken up and reduced. So the heterogeneous nuclei
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increased and the moderate agitation is unable to decrease them. Thus the grains are much more refined
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from 31.4 μm to 15.6 μm in average size and the yield stress has an effectively improvement from 95 MPa to 114 MPa. Comparing the situation of stirring at 540 rpm with stirring at 360 rpm, the violent
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agitation not only eliminate the large agglomerations but also reduce the heterogeneous nuclei by greatly promotion of the reaction (3). So the grains grow to 30.2 μm in average size. But more uniformly
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dispersed TiB2 particles contribute to stronger Orowan strengthening after elimination of large
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agglomerations. Thus the yield stress still has a little improvement from 114 MPa to 120 MPa.
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The improvement of UTS of all composites may be attributed to the gain boundary strengthening compared with matrix metal. When gathering along the grain boundary, TiB 2 particles cannot contribute
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to Orowan strengthening. But they can directly hinder the dislocation slip since they have high elastic modulus and high hardness. The CTE (Coefficient of Thermal Expansion) mismatch between these
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particles with matrix also brings dislocation strengthening at grain boundary, according to CTE mismatch strengthening [1, 31].The extremely tiny particles fill in the gaps of large particles and improve the gain boundary strengthening. Comparing the situation of stirring at 180 rpm with no stirring, small agglomerations are reduced and more TiB2 particles uniformly distribute along the grain boundary. So the UTS is still a little improved with a slightly bigger average grain size. Comparing the situation of stirring at 360 rpm with stirring at 180 rpm, the much more refined grains greatly increase the grain boundary regions. And uniformly distributed particles along the grain boundary contribute to an obviously improvement of UTS from 223 MPa to 251 MPa. Comparing the situation of stirring at 540 rpm with
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stirring at 360 rpm, although the distribution of TiB 2 particles along the grain boundary becomes more
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uniform, the bigger grains decreases the grain boundary regions and affects the UTS. The UTS is slightly
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lower. The elimination of large agglomerations and the uniform distribution of TiB2 particles along the
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grain boundary effectively improve the UTS, as the UTS of the composite with stirring at 540 rpm is improved by 20 MPa compared with the composite with stirring at 180 rpm, which have a similar average
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grain size.
The ductility of all composites is lower than the matrix metal due to the hindering of TiB 2 particles
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on the deformation of grains. But with uniform distribution of TiB 2 particles and reducing of large
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CONCLUSIONS
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agglomerations, the ductility can be improved.
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The 5vol% TiB2/Al-4.5Cu composites are successfully prepared with mechanical stirring. TiB2 particles are uniformly distributed without cluster or large precipitate of intermediate phases Al3Ti and
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AlB2. The size of TiB2 particles is under 0.4 μm in diameter. The grains are effectively refined. With the mechanical stirring below the interface of melt and salts during the reaction holding, the introduction of by-products like Al2O3 or K3AlF6 into the composites is effectively avoided. The large agglomerations are effectively reduced by mechanical stirring and finally eliminated when the speed of mechanical stirring rises to 540 rpm. The distribution of TiB2 particles becomes more uniform with the increasing of stirring speed.
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The formation of agglomerations along the grain boundary without clear particulate contours is
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explained. The extremely tiny particles fill the gaps of large particles with the Al2Cu phase which wet
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the TiB2 particles.
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The as-cast mechanical properties of TiB2/Al-4.5Cu composites have a great improvement compared with the matrix metal. The yield stress of composites has been improved by 26 %, 46 %, 75 % and 85 %
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with the mechanical stirring at 0, 180, 360 and 540 rpm, respectively, and the improvements of UTS is
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28 %, 34 %, 51 % and 46 %, respectively.
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AKNOWLEDGMENTS
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This work was financially supported by the National Basic Research Program of China (No. 2012CB619600), the Fundamental Research Funds for the Central Universities, HUST (No.
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2014QNRC003, No. 2015QN010), and the Research Project of State Key Laboratory of Materials Processing and Die & Mould Technology. The authors would also like to express their appreciation to
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Analytical and Testing Center, HUST.
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Graphical abstract
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Highlights
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1. No by-products were introduced into the melt by mechanical stirring below interface.
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2. Large agglomerations were eliminated in high TiB2 volume percentage composites.
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3. Tiny TiB2 particles with a size of 10-100nm were found, and analyzed by TEM.
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